This application relates to optical fiber devices, and more specifically, to techniques for coupling optical pump light into a double-clad fiber device and devices employing the same.
An optical fiber-may be designed to operate as an optical gain medium. In general, the core of such an optical fiber may be doped with certain active ions such as erbium and other rare earth ions to produce the desired optical gain by receiving optical pump light at a desired pump wavelength. Hence, when optically pumped at the pump wavelength (e.g., near 0.98 micron), the doped fiber core absorbs the pump light to produce light at an emission wavelength different from the pump wavelength (e.g., near 1.55 micron). Such a doped-fiber may be used to form various fiber devices, including but not limited to, a fiber laser for producing a laser at the emission wavelength and a fiber optical amplifier to amplify an optical signal at the emission wavelength.
The above doped fiber may be made in a double-clad configuration which includes an inner cladding layer to surround the fiber core and to have an index of refraction less than that of the fiber core. This inner cladding layer forms a cladding optical waveguide along the fiber that supports multiple modes and has a cross section area much greater than that of the fiber core to achieve a large numerical aperture. There may be least one outer cladding layer with an index of refraction less than that of the inner cladding layer formed outside the inner cladding layer. In operation, the pumplight is first coupled into the inner cladding layer to propagate in the cladding wave guide along the fiber. The pump light in the inner cladding layer then interacts with the fiber core and is absorbed by the doped ions. Since the pump is coupled into the core through the fiber cladding, the double-clad fiber is also referred to as a cladding-pumped fiber.
The cladding pumping can be efficient since the pump light is coupled through the large cross section of the inner cladding layer. High-power multi-mode pump sources may now be used to produce pump light for the double-clad fiber devices. In addition, the absorption of the pump light by the fiber core is extended over the interface between the inner cladding layer and the fiber core. Therefore, a high pump power level may be achieved in the fiber core without exceeding the damage threshold intensity for the fiber core.
This application includes techniques and devices that optically couple multiple pump fibers and a signal input fiber to a double-clad fiber with a high coupling efficiency.